Nowadays, pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually not outside the temperature range from about -40.degree. C. to about 100.degree. C. Such pipes are now preferably made of polyolefin plastic, such as ethylene plastic (HDPE, MDPE), or of polyvinyl chloride (PVC) or alternatively of other materials that are not necessarily based on polymer.
A drawback of such pipes at low temperatures is that if they are subjected to stress which initiates a crack, this crack can be spread or propagated very fast a considerable distance in the pipe and cause catastrophic failure of the pipe. The speed of propagation of the crack can be as high as about 100-300 m/s in the longitudinal direction of the pipe. This type of cracking is generally called rapid crack propagation (RCP), which is a generic term. To make a pressure pipe of plastic material acceptable, it should, at a certain temperature and a certain inner pressure, not have rapid crack propagation greater than 4 times the pipe diameter in the longitudinal direction of the pipe.
The RCP properties of a given pipe can be determined in various ways. According to one method, called the S4 test (Small Scale Steady State), which has been developed at Imperial College, London, and which is described in ISO DIS 13477, a pipe is tested, which has an axial length not below 7 pipe diameters. The outer diameter of the pipe is about 110 mm or greater and its wall thickness about 10 mm or greater. When determining the RCP properties of a pipe in connection with the present invention, the outer diameter and the wall thickness have been selected to be 110 mm and 10 mm, respectively. While the exterior of the pipe is at ambient pressure (atmospheric pressure), the pipe is pressurised internally, and the internal pressure in the pipe is kept constant at a pressure of 0.5 MPa positive pressure. A number of discs have been mounted on a shaft inside the pipe to prevent decompression during the tests. A knife projectile is shot, with well-defined forms, towards the pipe close to its one end in the so-called initiating zone in order to produce a rapidly running axial crack. The initiating zone is provided with an abutment for avoiding unnecessary deformation of the pipe. The test equipment is adjusted in such manner that crack initiation takes place in the material involved, and a number of tests are effected at varying temperatures. The axial crack length in the measuring zone, having a total length of 4.5 diameters, is measured for each test and is plotted against the measured temperature. If the crack length exceeds 4 diameters, the crack is assessed to propagate. If the pipe passes the test at a given temperature, the temperature is lowered successively until a temperature is reached, at which the pipe no longer passes the test, but the crack propagation exceeds 4 times the pipe diameter. The critical temperature (T.sub.crit) is the lowest temperature at which the pipe passes the test. The lower the critical temperature the better, since it results in an extension of the applicability of the pipe. It is desirable for the critical temperature to be as low as possible.
In this context, it should be mentioned that some plastic materials have so poor RCP properties (high T.sub.crit) that the value of T.sub.crit exceeds the normal measuring limit for T.sub.crit which is +23.degree. C. In such cases, use is conventionally made, for the purpose of the present invention, of a T.sub.crit value of +23.degree. C. for these materials.
For bimodal polyethylene plastic intended for pipes that are required to be resistant to RCP, T.sub.crit generally is in the range of from about 0.degree. C. to about -25.degree. C., whereas T.sub.crit for a conventional polypropylene homopolymer is above about +23.degree. C.
Another method of determining the RCP properties for a pressure pipe of plastic is to determine the critical pressure, P.sub.crit, instead. The method conforms with the above-described method, but instead of keeping the pressure constant and successively lowering the temperature, the temperature is kept constant at 0.degree. C..+-.2.degree. C. and the pressure sure in the pipe is successively increased. The highest pressure at which the pipe still passes the test, is called the critical pressure, P.sub.crit The method for determining P.sub.crit is less preferred in connection with the development of materials, since the tested pipe must usually be subjected to such high pressures as 1 MPa or more, which involves risks and difficulties. Therefore, the method of determining T.sub.crit is preferred and used in the present invention.
It would mean a great advantage if it is was possible to provide pipes of polypropylene material having improved resistance to rapid crack propagation, RCP. It would be particularly advantageous if it was possible to provide pressure pipes of polypropylene material having a T.sub.crit below 0.degree. C., preferably below -5.degree. C.